if you know anybody, who already has google+, point him to this: http://eu.techcrunch.com/2011/06/30/how-to-hack-google-to-se... the last described method works just fine.

or in short: add people you want to invite to a circle, klick the share "button" on the upper right hand, share to cirlce, make sure to check "Also email XX people not yet using Google+"